The present invention relates to microwave regenerated filters for use in exhaust streams for capturing particulate matter. In particular the present invention relates to microwave regenerated porous ceramic diesel exhaust filters having improved resistance to melting during regeneration.
Recently much interest has been directed towards the diesel engine due to its efficiency, durability and economical aspects. However, diesel emissions have come under attack both in the United States and Europe, for their harmful effects on the environment and on humans. As such, stricter environmental regulations will require diesel engines to be held to the same standards as gasoline engines. Therefore, diesel engine manufacturers and emission-control companies are working to achieve a diesel engine which is faster, cleaner and meets the most stringent of requirements under all operating conditions with minimal cost to the consumer.
One of the biggest challenges in lowering diesel emissions is controlling the levels of diesel particulate material (DPM) present in the diesel exhaust stream. In 1998 DPM was declared a toxic air contaminant by the California Air Resources Board. As mentioned herein above legislation has been passed that regulates the concentration and particle size of DPM pollution originating from both mobile and stationary sources.
DPM which is mainly carbon particulates, is also known as soot. One way of removing diesel soot from the diesel exhaust is through diesel traps. The most widely used diesel trap is the diesel particulate filter (DPF) which is used to capture the soot. The DPF is designed to provide for nearly complete filtration of the soot without hindering the exhaust flow. However, as diesel soot accumulates, exhaust flow becomes increasingly difficult and the DPF must either be replaced or the accumulated diesel soot must be cleaned out. Cleaning the accumulated diesel soot from the DPF is achieved via burning-off or oxidation to CO.sub.2 and is known in the art as regeneration. Regeneration is considered to be a superior approach over DPF replacement since no interruption for service is necessary.
The regeneration process can be either passive or active. In a passive system, regeneration occurs when the DPF becomes so filled with carbon particulates that heat accumulated in the exhaust system due to excessive back pressure raises the temperature of the carbon to a point where it ignites. This design can result in thermal shock or melt down of the filter, high fuel penalty and poor filtering action. Active regeneration is considered to be a superior approach over passive regeneration. In an active system, heat required to initiate combustion of the soot is generated by an outside source. Electrical power, fuel burners and microwave energy have all been studied as heat sources. Microwave energy is considered to be a superior approach over electrical power and fuel burners because it is highly efficient, cost effective and energy saving.
Microwave regeneration has been addressed, for example in U.S. Pat. No. 5,087,272 (Nixdorf) and Japanese Pat. Appl. Disclosure No. 6-241022 (Arakawa).
Nixdorf discloses a microwave regenerated filter made of single crystal silicon carbide whiskers which are consolidated into a preform of cylindrical configuration or into a thin layer such as a paper, which is folded into a tortuous structure.
Japanese Pat. Appl. Disclosure No. 6-241022 (Arakawa) discloses a filter having an electromagnetic absorbing coating which in combination with a regeneration gas acts to ignite the particles trapped by the filter. The coating materials disclosed are silicon carbide, aluminum nitride, titanium oxide and zinc oxide whiskers.
Two important problems exist with regeneration, and both are consequences of the carbon soot combustion which is highly exothermic. Rapid heating and cooling generates thermal stress that overtime could cause cracking of the DPF. Even worse, if too much carbon soot accumulates in the DPF, the regeneration process will evolve such a large quantity of heat that the DPF melts. To avoid these problems, the entire regeneration process should be performed as often as necessary to ensure that the amount of accumulated soot is insufficient to generate a large exotherm that could crack or melt the filter, thereby maintaining peak DPF performance.
Standard commercially available filters are made of cordierite (2MgO-2Al.sub.2 O.sub.3 -5SiO.sub.2) which has a low coefficient of thermal expansion (.about.0.6-1.0.times.10.sup.-6 /.degree. C.) and is therefore suitable for applications involving rapid heating and cooling. However, cordierite has a relatively low melting point (.about.1458.degree. C.). Hence, during the regeneration process cordierite filters are prone to localized melting as opposed to cracking since heat cannot be easily transferred through the entire body. Further, cordierite is transparent to microwaves.
A need therefore exists for a filter for trapping and combusting diesel exhaust particulates which can be regenerated by microwave energy, which is cost-effective and highly efficient and which has good strength and high thermal shock resistance to withstand the harsh chemical and physical conditions of typical diesel exhaust streams.
It is the purpose of the present invention to provide such a filter.